Electrical and electrochemical properties of
Li
2
M(WO
4
)
2
(M ¼ Ni, Co and Cu) compounds
Karim Karoui, *a
Abdelfattah Mahmoud,bAbdallah Ben Rhaiema
and Fr`ed´eric Boschinib
Li2M(WO4)2(M¼ Co, Cu or Ni) materials have been synthesized using the solid-state reaction method. X-ray diffraction measurements confirmed the single phase of the synthesized compounds in the triclinic crystal system (space group P1). The SEM analyses revealed nearly spherical morphology with the particle size in the range of 1–10 mm. The IR spectra confirm the presence of all modes of WO42. The impedance spectroscopy measurements showed the presence of grain boundaries and allow determination of the conductivity of the synthesized materials at room temperature. As positive electrode materials for lithium ion batteries, Li2M(WO4)2(M¼ Co, Cu or Ni) cathode materials deliver initial discharge capacities of 31, 33 and 30 mA h g1for cobalt, nickel, and copper, respectively.
1.
Introduction
In recent years, tungsten (W) based electrode materials such as oxides, nitrides, and oxysalts have been proven to be feasible for electrochemical energy storage, thanks to their high specic capacity, good working voltage, abundant reserves, and envi-ronmental friendliness. In addition, metal tungstate as an independent entity, such as FeWO4, MnWO4 and ZnWO4, reveals excellent electrochemical performance on account of the synergistic effect of different metal elements.1–5Among them, cobalt tungstate (CoWO4) and nickel tungstate (NiWO4) have been researched as promising replacements for catalysis.6
Double tungstates materials have been considered to be attractive materials basically thanks to their interesting lumi-nescence properties and possible application in the eld of solid-state lasers.7–9 These materials have a general formula A2M(WO4)2where A is a monovalent cation and M (Ni, Co and Cu.) is a divalent metal. Two different coordination have been found for tungsten: (i) tetrahedral for the scheelite-like double tungstates as in NaLa(WO4)2(ref. 10) or (ii) octahedral for those adopting the wolframite type of structure as in LiFe(WO4)2.11 Nevertheless, a few double tungstates have been structurally investigated and not much has been study about their chemical and physical properties. The wolframite type structure is very
common among MWO4 compounds (where M is a transition
metal), and it can be described as made up of hexagonal close-packed oxygens with certain octahedral siteslled by M2+and W6+ cations in an ordered way.12 As though the available
information about this type of tungstates remains scarce, we explored the existence of new lithium metal(II) double tung-states and to investigate the electrical and electrochemical properties for which the previously published information was incomplete. The aim of this work is, to investigate and study the electrical and electrochemical properties of Li2M(WO4)2(M ¼ Ni, Co and Cu) compounds at room temperature. We report here for therst time the electrochemical tests of Li2M(WO4)2as positive electrode material for Li-ion batteries and we investi-gate the electrical and SEM properties at room temperature to combine the electrochemical properties and conductivity measurements with morphological properties.
2.
Experimental
In a typical procedure, polycrystalline Li2M(WO4)2(M¼ Ni, Co and Cu) powder sample was prepared using mixtures of stoi-chiometric amounts of high purity (Sigma Aldrich) ($99%) NiO, ZnO, CuO, Li2CO3, and WO3via conventional solid state reac-tion method.
Stoichiometric amounts of the starting reagents were mixed and intimately ground in an agate mortar. An excess of lithium was used during these preparations to compensate for the evaporation of the latter at high temperature. Initially the samples werered at 550C under air for 60 hours to allow decarbonation and then between 650 and 700C for 160 hours as summarized in Table 1.
Table 1 Conditions of synthesis of Li2M(WO4)2(M¼ Co, Ni and Cu)
Sample Synthesis Color
Li2Ni(WO4)2 550C/60 h, 700C/160 h Yellow Li2Cu(WO4)2 550C/60 h, 700C/160 h Green-yellow Li2Co(WO4)2 550C/60 h, 650C/146 h Purple
aLaboratory of Condensed Matter, Faculty of Science of Sfax, University of Sfax,
BP1171– 3000 Sfax, Tunisia. E-mail: karouikarim36@yahoo.com
bGREENMAT, CESAM Research Unit, University of Liege, Chemistry Institute B6,
Quartier Agora, All´ee du 6 aoˆut, 13, B-4000 Liege, Belgium Cite this: RSC Adv., 2019, 9, 6785
Received 15th December 2018 Accepted 16th February 2019 DOI: 10.1039/c8ra10306b rsc.li/rsc-advances
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Aer heat treatment, samples have been grounded and then reheated to ensure homogeneity. The colors of these synthesized samples, which depend on the used metal, are in good agreement with the reported colors in the literatures as indicated in Table 1. The phase purity has been examined and conrmed using powder diffraction. Powder X-ray diffraction (XRD) data were collected with a Bruker D8 ECO powder diffractometer using Cu Ka radiation (l ¼ 1.5418 ˚A), operating from 2q ¼ 15 to 80. The crystal structure was rened by the Rietveld method, starting from the observed powder diffraction pattern and using Full-prof soware. The morphology and particle size were studied with a scanning electron microscope (XL 30 FEGESEM, FEI) with an accelerating voltage of 15 kV under high vacuum. Samples were deposited on carbon tapes. Sputtering deposition was done with gold target under argon atmosphere (Balzers, SCD004, Sputter coater).
To determine the particle size distribution, at least 10 micrographs were taken for each LTO-sample in different regions of the holder, and about 100 particles were analyzed using the commercial soware IMAGEJ.13
The IR measurements were performed using a Perkin Elmer 1600FT spectrometer. Samples were dispersed with spectro-scopic KBr and pressed into a pellet. Scans were run over the range 400–4000 cm1at room temperature.
The ATG analyze was carried out using a thermogravimetric balance TGA-50 Shimadzu in the temperature range from 30 to 600C under O2atmosphere with a heating rate of 10C min1. The real (Z0) and imaginary (Z00) parts of electrical impedance data were measured by means of Solartron1260A Impedance Analyzer coupled with a 1296A Dielectric Interface in the 1– 106 Hz frequency range at room temperature. This measure-ment is realised on pellet disks of about 8 mm in diameter and 1.2 mm in thickness.
The working electrodes were prepared by dispersing 60 wt% active material (complex), 20 wt% conductive carbon, and
20 wt% binders (polyvinylidene uoride) in
1-methyl-2-pyrrolidinone (NMP) to stir into homogeneous slurry during 2 hours. The slurry was coated on aluminum foil, dried at 110C in a vacuum oven for 8 h. The active material loading of elec-trodes was in the range of 1–2 mg cm2. The electrochemical properties were studied using coin cells (2032, R-type), which were assembled in the Ar-lled glove box. Lithium metal (Aldrich) was used as counter and reference electrode, 1 M LiPF6 in ethylene carbonate and dimethylcarbonate (1/1, v/v) as elec-trolyte solution and celgard as separator.
The cyclic voltammetry (CV) analysis was performed in the range of 1.5–4.5 V at 0.1 mV s1. The galvanostatic charge/ discharge curves were measured using a multichannel Bio-logic potentiostat (VMP3) in the voltage range 1.5–4.5 V vs. Li+/ Li0at C/20 rate.
3.
Result and discussions
3.1. X-ray powder
X-ray diffraction patterns, depicted in Fig. 1, corresponding to Li2M(WO4)2for cobalt, nickel, and copper, respectively, show that the samples are single phase. Indeed, all the peaks can be
indexed in the unique triclinic unit cell and the P1 space group. The crystal structures of Li2M(WO4) (M ¼ Ni, Co and Cu) materials were rened using Rietveld method. The obtained results of the Rietveld renement are summarized in Table 2. The obtained results are in good agreement with the data re-ported in the literature.14The XRD patterns of Li
2Co(WO4)2and Li2Ni(WO4)2 are very similar, suggesting that the two compounds are isostructural. It is worth pointing out that the main difference between the structures of Li2M(WO4)2(M¼ Co or Ni) and Li2Cu(WO4)2 is the relative positions of the atoms and the symmetry elements. This seems to be due, most likely,
Fig. 1 Powder X-ray diffraction pattern and Rietveld refinement of Li2M(WO4)2(M¼ Co, Ni and Cu).
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to the strong Jahn–Teller effect found in the CuO6octahedra in which an important elongation is produced.14
3.2. Particle morphology
The morphology and particle size of the electrode materials for rechargeable batteries strongly affect the Li+ de-/insertion reaction. The morphological study was conducted by scanning electron microscopy (SEM) to perform an observation of the morphology and also for the determination of the particle size distribution of the Li2M(WO4)2for cobalt, nickel, and copper compounds. Fig. 2a, 3a and 4a show the SEM micrographs of Table 2 System, space group and the unit cell parameter of
Li2M(WO4)2(M¼ Co, Ni and Cu)
Sample System/space group Parameter
Li2Ni(WO4)2 Triclinic/P1 a¼ 4.903/b ¼ 5.597/c ¼ 5.836 a ¼ 70.881/b ¼ 88.548/g ¼ 115.436 Li2Cu(WO4)2 Triclinic/P1 a¼ 4.962/b ¼ 5.492/c ¼ 5.884 a ¼ 70.741/b ¼ 85.995/g ¼ 66.041 Li2Co(WO4)2 Triclinic/P1 a¼ 4.916/b ¼ 5.660/c ¼ 5.875 a ¼ 69.491/b ¼ 91.450/g ¼ 116.153
Fig. 2 (a) SEM micro-graphs and (b) the corresponding histograms for statistical calculation of the particle size of Li2Co(WO4)2.
Fig. 3 (a) SEM micro-graphs and (b) the corresponding histograms for statistical calculation of the particle size of Li2Ni(WO4)2.
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the synthesized materials. This morphological features, together with the X-ray data (Fig. 2) conrms the high crystal-linity of the studied samples, The comparison of the micro-graphs shows clearly that the metal based Li2M(WO4)2(M¼ Co, Ni, Cu) oxides materials has a great inuence on the morpho-logical properties. Strong agglomeration of the primary parti-cles is observed mainly for the material Li2Co(WO4)2 (Fig. 5) which could have an effect on the conductivity and electro-chemical properties of the Co-based compound.
The analysis of SEM images using the soware (image J) leads to the determination of the particle size distribution of the elementary particles. The statistical determination of the sizes was carried out on a hundred of the particles. Fig. 2b, 3b and 4b shows the histogram of the statistical calculation of the particle size. The size distribution of the particles is of Gaussian type and is relatively homogeneous with an average particle size of about 2.1 mm, 3.46 mm, and 9.81 mm for the Li2Co(WO4)2,
Li2Ni(WO4)2, and Li2Cu(WO4)2respectively. This result conrm the SEM micro-graphs (Fig. 3) and indicates that the nature of the metal has a strong effect on the average particle size and the particle size distribution of the primary particles of Li2M(WO4)2 materials.
Fig. 4 (a) SEM micro-graphs and (b) the corresponding histograms for statistical calculation of the particle size of Li2Cu(WO4)2.
Table 3 IR frequency and assignment of Li2M(WO4)2(M¼ Co, Ni and Cu)
IR frequency (cm1)
Symmetry and assignments Li2Ni(WO4)2 Li2Cu(WO4)2 Li2Co(WO4)2
915 908 910 Au ns(W–O) 826 816 822 Bu nas(W–O) 751 748 750 Au nas(WOOW) 594 581 584 Bu nas(WOOW) 502 486 493 Au nas(WOOW) 454 450 450 Bu nas(WOOW)
Fig. 5 The SEM micrographs with high magnification of Li2Co(WO4)2.
Fig. 6 Infrared absorption spectra of Li2M(WO4)2(M¼ Co, Ni and Cu).
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3.3. Infrared spectroscopy
The IR spectra are presented in Fig. 6. Table 3 summarizes the observed vibrational wavenumbers and their assignments. A look at the spectra allows makes it possible to notice that these spectra are very similar and the frequencies of all the peaks observed are very close suggesting that these compounds are isostructural. The assignment of the internal modes are ob-tained by a comparison with previous studies compounds ZnWO4, LiIn(WO4)2, LiFe(WO4)2 and LiYb(WO4)2.15 This attri-bution shows the all modes observed in experimental spectra are assigned to the symmetric and asymmetric vibrational stretching of (WO4)2and conrm the formation of the double oxygen bridges in these structures,16the deformation modes of WO4and the internal modes of MO6which M¼ Ni, Cu and Co are located below 400 cm1.
3.4. Thermal analyze
The thermal analysis of Li2MWO4 (M ¼ Ni, Cu and Co) was carried out for the as prepared samples using thermogravi-metric analysis (TGA) to determine the thermal stability and crystallization temperature of these compounds. Fig. 7 shows the TG curve of as-prepared s materials. From the TGA curve, it is evident that this compound undergoes a very low weight loss (between 100% and 99%) observed between 30 and 600C. The weight loss observed below 150C is associated with the solvent evaporation, whereas the major weight loss over the range of 150–600C is essentially ascribed to the precursor decomposi-tion and evaporadecomposi-tion of other organic funcdecomposi-tional groups.17 A larger loss above 550C for the cobalt-based compound indi-cates the existence of carbonates.
3.5. Conductivity at room temperature
Fig. 8 presents the Nyquist plot of Li2M(WO4)2for cobalt, nickel, and copper, respectively at room temperature. The Nyquist plot consists of a depressed semicircle in the high-frequency region and inclined line in low frequency region. The depressed semicircle in middle to high frequency region is attributed to
the response of the grain and the inclined line in low frequency region corresponding to the response of the grain boundary and electrode effect.18,19The three sample are described by the same Fig. 7 TGA curve of as-prepared Li2M(WO4)2 (M¼ Co, Ni and Cu)
compounds.
Fig. 8 Complex impedance spectra as a function of temperature with electrical equivalent circuit of Li2M(WO4)2(M¼ Co, Ni and Cu).
Table 4 Conductivity of Li2M(WO4)2(M¼ Co, Ni and Cu)
Sample Conductivity at 300 K
Li2Ni(WO4)2 1.46 108U1cm1
Li2Cu(WO4)2 1.17 108U1cm1
Li2Co(WO4)2 0.29 108U1cm1
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equivalent circuit which is modeled by a combination series of two cell, therst is composed a parallel R and CPE circuits and the second is composed a parallel R, C and CPE elements, where R is resistance (bulk resistance), C is capacitance and CPE is the complex element: constant phase element (capacity of the fractal interface).20The CPE element accounts for the observed depression of semicircles and also the non-ideal electrode geometry. The obtained values of the simulate Nyquist plots allows to determinate the conductivity of grain based in the following equation:21
sg¼ e RgS
The calculate value of conductivity are listed in Table 4 which indicate the weak conductivity of these materials at room temperature. We can also notice that the highest conductivity among the three synthesized compounds is attributed to the Ni-based compound and the lowest value attributed to the Li2 -Co(WO4)2. This result can be explain by the agglomeration observed in the SEM micro-graphs of the Li2Co(WO4)2 compound (Fig. 3) and not observed in the SEM micro-graphs of the of Ni and Cu based compounds.
3.6. Electrochemical properties
In order to compare the electrochemical properties of the high purity Li2M(WO4)2 oxides synthesized in this work. The elec-trochemical performances of three Li2M(WO4)2(M¼ Co, Ni, Cu) samples were evaluated as positive electrode in coin cells in voltage range 1.5–4.5 V at room temperature. The electro-chemical properties of all samples were evaluated without any
further optimization such as particle size reduction or carbon coating.
Cyclic voltammetry (CV) is a convenient technique for eval-uation of the electrochemical performance and electrode kinetics of oxide material. Fig. 9 displays the CV curves of Li2M(WO4)2for cobalt, nickel, and copper at room temperature in the voltage range of 1.5–4.5 V at scan rate of 0.1 mV s1. The CV curves show only one pair symmetrical and large redox peaks corresponding to the intercalation/deintercalation of Li+ ions into/from the structure of Li2M(WO4)2 materials. The anodic and cathodic peaks occur at about 4.35 and 1.8 V, respectively. To determine the electrochemical reaction revers-ibility, an important parameter of the separation potential (DEp) between anodic and cathodic peaks should be taking into account. The DEp of the Li2Co(WO4)2 is 2.15 V, for the Li2 -Cu(WO4)2is 1.98 V and for the Li2Ni(WO4)2is 1.93 V. This result conrm that the material Li2Ni(WO4)2 shows the best revers-ibility of lithium extraction/insertion during charge/discharge process.
To study the inuence of the nature of metal of Li2M(WO4)2 materials on their cycling performances, The electrode mate-rials were galvanostatically evaluated in the voltage range of 1.5– 4.5 V at room temperature.
The electrochemical charge/discharge measurements were carried at the charge/discharge rate C/20 for the Li2Ni(WO4)2, Li2Co(WO4)2 and Li2Cu(WO4)2 compounds. Initial charge/ discharge curves recorded at room temperature are presented in Fig. 10 for the studied cathode materials. The three materials show a continuous increase of the voltage with Li-extraction (charge), than a pseudo plateau starting from 4.35 V with low polarization is observed for all samples, which corresponds to
Fig. 9 The cyclic voltammetry curves of Li2M(WO4)2(M¼ Co, Ni and Cu) cathode materials in the voltage range 1.5–4.5 V.
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approximately 0.1–0.2 Li+-ion de-insertion during the charge processes. During the discharge, the reversible process is observed. The voltage decreases continuously until the appearance of small plateau at around 1.8 V in good agreement with the CV measurement. The analysis of the charge/discharge curves suggests the existence of the of a solid solution domain during therst charge and discharge process of Li2Ni(WO4)2, Li2Co(WO4)2and Li2Cu(WO4)2materials.22–24Fig. 11 shows the evolution of the discharge capacity as function of the number of cycles during 50 cycles at C/20. Li2Ni(WO4)2, Li2Co(WO4)2and Li2Cu(WO4)2materials show initial discharge capacities of 35, 32 and 30 mA h g1respectively. However the three materials show different cycling behaviors. Noticeable capacity drop until the 15th cycle is observed Li2Co(WO4)2and Li2Cu(WO4)2 elec-trode materials. However, Li2Ni(WO4)2electrode material shows stable cycling during 15 cycles with a reversible discharge capacity of about 30 mA h g1. This rapid decrease in capacity for Li2Co(WO4)2 and Li2Cu(WO4)2 materials is mainly due to their high particle size and to the strong agglomeration of the particles of these materials and was conrmed by the results obtained by SEM analyses. Li2Ni(WO4)2, Li2Co(WO4)2 and Li2 -Cu(WO4)2materials deliver discharge capacity of about 23, 13 and 11 mA h g1respectively aer 50 cycles. As shown, Aer 50 cycles, the sample Li2Ni(WO4)2shows the best cycling behavior. This electrode exhibits the highest rechargeable capacity (30 mA h g1) during 15 cycles, with highest capacity retention aer 50 cycles (approximately 50% of capacity retention).
It is easily understood that the capacity drop aer rst discharge for the samples based on Co and Cu should be related to their high particle size which leads to the unfavorable interaction with liquid electrolyte.25–27 Interestingly, Li2 -Ni(WO4)2 illustrates the best capacity retention compared to Li2Co(WO4)2 Li2Cu(WO4)2 during cycling. Indeed, among the studied samples, the Li2Ni(WO4)2 shows the best electro-chemical performances due to its good crystallinity, the small-est particle size which benets Li+-diffusion in Li
2Ni(WO4)2 structure.
4.
Conclusions
Li2M(WO4)2 for cobalt, nickel, and copper pure compounds with wolframite type were successfully synthesized by a solid-state reaction method. The powders have a triclinic structure with P1 space group and the particle size is in the range 1–10 mm. IR spectra show that these compounds are similar and conrm the presence of the WO42. Complex impedance anal-ysis reveals the contribution of the bulk grain boundary in the electrical properties and allows to determinate the equivalent circuit. The conductivity of these compounds was determined and showed the insulating nature at room temperature. The cyclic voltammetry measurements suggest that the separation potential (DEp) between anodic peak and cathodic peak is high which explains the difficulty to use the studied compounds as a cathode material for Li-ion batteries battery systems. The electrochemical charge/discharge measurements suggest that material based on Ni metal provides the enhanced specic capacity (35 mA h g1) in good agreement with the result obtained by the SEM results indicate the low particle size of the Li2Ni(WO4)2compound that present low agglomeration of the primary particles and impedance spectroscopy results that conrmed that Li2Ni(WO4)2material has the high conductivity. Fig. 10 Selected voltage profiles vs. capacity of Li2Ni(WO4)2, Li2
-Co(WO4)2and Li2Cu(WO4)2cathode material for Li-ion batteries in the voltage range 1.5–4.5 V at C/20.
Fig. 11 Evolution of the discharge capacity vs. cycle number the of Li2M(WO4)2(M¼ Co, Ni and Cu) cathode materials for Li-ion batteries in the voltage range 1.5–4.5 V at C/20.
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Con
flicts of interest
There are no conicts to declare.References
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